The “dead time” of a measurement instrument, such as an oscilloscope, is a time period during which data acquisition circuitry does not respond to a valid trigger event because the oscilloscope is busy performing other tasks and so is not able to process trigger events that may occur. Consequently, a waveform representing an electrical signal being monitored is not displayed for the missed valid trigger event. In an analog oscilloscope, for example, dead time occurs during the beam retrace time on a cathode ray tube. In a digital oscilloscope, dead time often occurs when the instrument is busy reading data from an acquisition memory associated with a previous acquisition, or busy drawing the acquired processed data to produce an image of the waveform for display.
Circuits under test often operate at rates much faster than a standard digital oscilloscope can display the corresponding waveforms. In fact, the typical digital oscilloscope “ignores” most trigger events because it is busy processing and drawing waveforms relating to data acquired in response to a prior trigger event. It is an unfortunate fact that such electronic circuits under test occasionally work in an unexpected manner. Occurrences of incorrect operation of the circuits under test may be rare, perhaps occurring once in thousands of correct cycles of operation. Thus, the oscilloscope may not acquire data representing waveforms that exhibit the incorrect operation of the circuit under test, i.e., an anomaly, because the oscilloscope may be busy at the instant that the anomaly occurs. An oscilloscope user may have to wait a long time in order to view the incorrect operation. Since only a small fraction of the waveforms are drawn on the oscilloscope display, failure to observe the incorrect operation cannot give the user confidence that the circuit under test is operating properly.
The basic digital oscilloscope has an architecture in which data is received and stored in an acquisition memory, and then acquisition is halted by a trigger event after a defined post-trigger interval. The acquired data then is read from the acquisition memory for processing and waveform drawing on a display before the acquisition system is again enabled to respond to new trigger events.
Co-pending U.S. patent application Ser. No. 11/388,428, filed by Steven Sullivan et al on Mar. 24, 2006 entitled “No Dead Time Data Acquisition”, now U.S. Pat. No. 7,652,465, is one attempt to enable the acquisition for display of data representing all trigger events. A measurement instrument receives a digitized signal representing an electrical signal being monitored and uses a fast digital trigger circuit to generate a trigger signal, wherein the trigger signal includes all trigger events within the digitized signal. The digitized signal is compressed as desired and delayed by a first-in, first-out (FIFO) buffer for a period of time (pre-trigger delay) to assure a predetermined amount of data prior to a first trigger event in the trigger signal. The delayed digitized signal from the FIFO is delivered to a fast rasterizer or drawing engine, upon the occurrence of the first trigger event, to generate a waveform image. The waveform image is then provided to a display buffer for combination with prior waveform images and/or other graphic inputs from other drawing engines. The contents of the display buffer are provided on a display screen at a display update rate to show a composite of all waveform images representing the electrical signal.
Two or more drawing engines may be used for each input channel of the measurement instrument to produce two or more waveform images, each waveform image having one of the trigger events at a specified trigger position within a display window. The waveform images are combined to form a composite waveform image containing all the trigger events for combination with the previous waveform images in the display buffer or with graphics from other drawing engines. For certain trigger positions within the display window, an indicator is provided to show that a trigger event may have been missed. Also, when there are no trigger events, a graphic of the signal content may still be provided for the display.
“No dead time” was defined as the ability for the user to see 100% of the trigger events that occur within an input signal on the display. Referring now to FIG. 1 locations A-H represent trigger events within the input signal. In this example, event C is used as a reference for drawing the data contained in the left frame, being placed at the trigger point within the graticule. The above-mentioned '428 U.S. patent application discusses a hold-off period within which trigger events are recognized, but not treated specially. These trigger events are shown on a display to an instrument user as part of the left frame, but do not form the basis for beginning new drawing cycles for these trigger events. Once the no dead time hold-off period is completed, the next trigger event is the focus for a new drawing cycle, in this example being event F which is located at the trigger point within the graticule for the right frame.
There are some problems with this method of defining “no dead time.” It is possible for the user to miss important and anomalistic information even while seeing 100% of the trigger events. As the trigger point is moved to the right hand side of the graticule, i.e., the pre-trigger region of the graticule is increased, more and more parallel drawing processes are required to keep up with the incoming trigger events that occur just past the right edge of the screen—practically there are limits to how far the trigger point may be moved toward the right edge of the graticule. Also drawing the same information more than once on screen may result in user confusion, i.e., the data around events D and E are drawn on both the left and right frames in the present example which are superimposed when drawn on a display screen, as shown in FIG. 1A.
As an example of missing important information, refer to FIG. 2 where the user is using edge triggering to look at individual pulses that form a longer pulse train. Reasonably, the user expects the instrument featuring no dead time acquisition to show any anomalies associated with each of the pulses. However this is not necessarily the case. Each of the trigger events, A-D, is shown in either the left or right frame. However, a glitch that is clearly visible in the pulse between trigger events B and C is not shown in either frame, i.e., it is not shown within the graticule on screen, as shown in FIG. 2A, and so is missed by the user. This condition occurs when the time between a first (A) and an Nth trigger event (C) is greater than the sum of the pre- and post-trigger time intervals that define the graticule; and the N−1 trigger event (B) is displayed when drawing the first trigger event. In this event a data “dead zone”, as opposed to a trigger event dead zone, may open up between the two processed frames into which information falls that is not drawn on the screen, i.e., in this example data within the post trigger time interval for the B trigger event but outside the pre-trigger interval for the C trigger event.
What is desired is a no dead time acquisition system that includes all trigger events and all data within the pre- and post-trigger time intervals for each of the trigger events.